The ability to transmit message information and power through the same cable lines has existed for some time. Telephone systems are prime examples of communications systems that transmit power and communications through the same wire bundle. In such a system, problems of interference and signal degradation are avoided by making the message information and power spectral frequencies to be far apart.
By way of example, in a typical telephone system a DC voltage of about 40 volts occupies the low frequency spectral range, while AC communications take place at much higher frequencies (e.g., in the kilohertz range). Various filtering techniques are then used to distinguish the two signals. Basically, this amounts to using a highpass filter to reject the power while detecting the message information, and a lowpass filter to reject the message while extracting the DC power.
Such prior art systems are not without their disadvantages. One major disadvantage of this type of prior art system lies in the fact that the transformer must be sized to handle the DC current without saturating. In general, a transformer which can accommodate DC currents without saturating has much poorer AC characteristics than one in which does not have to handle any DC current. These degraded AC characteristics are manifested by poor communications signal quality and by a limited bandwidth. Hence, the operating performance of the communications network is compromised in this type of conventional communications system.
To overcome the difficulties associated with providing power and communications along the same cable, some practitioners have chosen to provide separate conductors for power and message delivery. However, this approach has also suffers from certain serious drawbacks. For instance, in a multi-drop communication system (e.g., a system providing communications between a plurality of separate nodes) it is often a stringent requirement that the miswiring of a single node not disrupt communications between other nodes connected to the network. In other words, if a particular node is miswired such that one of the power lines has been inadvertently connected to a communications line, or visa versa, then the entire network should not fail. It should be noted that in multi-drop networks having many communications nodes, miswiring of individual nodes is relatively commonplace. Past approaches which separate their respective power and communications lines have failed to adequately safeguard against the eventuality of a miswiring condition.
A further problem of conventional power distribution approaches is that they tend to make inefficient use of cable. That is, the distance between communications nodes is often limited because of DC voltage drops experienced as power is distributed about the network. For a multi-drop system, this problem is particularly troublesome. Because the maximum distance between two given nodes is frequently limited by the DC voltage drop on the power distribution lines--rather than the ability of individual nodes to achieve reliable AC communications--the scope of prior art communications systems has been limited.
Therefore, what is needed is a means of providing power and communications over the same cable network which overcomes the problems described above. As will be seen, the present invention provides a wire-based communications network in which power and message information is delivered over the same cable network with improved AC characteristics. The enhanced communication capabilities of the present invention permit greater communication speeds and transmission over greater distances. In addition, the present invention allows for distribution of power and message information over a greater number of nodes. Moreover, the present invention solves the problem of network failure in the event that a single node is accidentally miswired.